Electron paramagnetic resonance probes of the radical decomposition of cumene hydroperoxide initiated by metmyoglobin

Abstract

Electron paramagnetic resonance studies have provided evidence for metmyoglobin initiation of the radical decomposition of cumene hydroperoxide, carried out in buffered aqueous solutions at ambient temperatures. The radicals formed oxidize aminopyrine to a free radical, readily detected at acidic pH, or react with the spin trap nitrosobenzene. The only species so trapped was the cumyl radical (optimal pH, 9.0), previously observed in a similar spin-trapping study of the chemical decomposition of cumene hydroperoxide in organic solvents. The earlier proposal that the cumyl radical arises from breakdown of an initially formed, unstable phenylcumyloxy nitroxide is consistent with the experimental findings of this study. Moreover, it was shown that the decomposition of cumene hydroperoxide initiated by ferrous ion or by other heme compounds occurs by the same mechanism. Thus, the very low peroxidatic activities of several hemeproteins with cumene hydroperoxide involve oxidizing free radicals, unlike H 2O 2-dependent oxidations catalyzed by true hemeprotein peroxidases, in which enzyme species are the functional oxidants.